Experiments have so far demonstrated that within a period of 6-12 hours after antigenic stimulation of the popliteal lymph node in sheep, cells appear in the lymph which show increased responses to in vitro stimuli and a proportion of them are proliferating due to a post-antigenic stimulus in the node as early as 2 hours after challenge. Coincident with the appearance of these cells are factors in the immune lymph and supernatants of these cells which provide nonspecific potentiation of in vitro responses of PBL to PHA. Additionally, the lymph contains true mitogenic factors. Both the mitogenic and potentiating activities fall within the molecular size ranges 20-40,000, 50-60,000, 120-140,000 and 180-200,000 daltons. The three lower molecular weight factors exhibit heterogeneity when examined by iso-electric focussing. These factors will be examined for their effects on the development of cytotoxic T cells, in vitro induction of antibody responses and the continual growth of thymocytes to establish any correlation with in vitro-produced factors in other animal systems. The kinetics of production of these and other helper/suppressor factors detected later in the response will be examined using infusion of 3H-leucine into the challenged node and subsequent collection of radiolabelled factors from efferent lymph and secreted factors from efferent cells on their exit from the infused node. The response to allogeneic and xenogeneic (human) cells will be analyzed by assaying the mixed lymphocyte reactivity and cytotoxic T cell development in the efferent population at various times following challenge with these antigens. This study, together with the isolation and characterization of regulatory factors produced during these responses will provide essential information on the contribution by specific and nonspecific mechanisms to the regulation of the immune response in vivo.